The present invention relates generally to power supply circuits such as regulated AC/DC and DC/DC converters, especially voltage supply regulation and specifically the fine regulation of partially regulated voltage supplies.
Voltage converters in power supplies often have more than one output. To save space and money it is not unusual that power supplies with several outputs fine regulate one of the outputs and only coarsely or slave regulate the other outputs. A disadvantage with coarsely regulated or slave regulated outputs is that they inherently have an undesirable load dependence, i.e. the output varies in dependence of the load. The load dependence of each of these outputs is a compound dependence comprising the load and load variation of the output in question and a cross dependant part from the loads and load variations of the other outputs. The cross load dependence is primarily caused by the regulated output's load and load variations. Due to this the specified load ranges of such power supplies are rated very conservatively to ensure that the different voltages at the outputs are within acceptable levels at all times.
This puts very strict restrictions on the use of such power supplies. Such restrictions can be acceptable at the time of construction, if the power loads are of a nature that only vary within the rated load range restrictions of the power supply. There will, however, more likely than not, be a problem when a construction/apparatus is modernized since a modernization frequently will involve, or be the primary reason for, changing high power consuming parts with more up to date low power devices. A power supply that is optimized for higher loads can in such cases become completely obsolete if it cannot keep its outputs within specified limits at the new load or loads. It is highly undesirable to completely change an otherwise functional power supply. Therefore there exists a need to be able to extend the load range of existing power supplies having unregulated, coarsely regulated, and/or slave regulated outputs. Power supplies designed today also commonly have to be able to supply one or more voltages within fairly narrow limits over a very wide load range due to, for example, power save functions which can cause one or more parts of an apparatus to go from a very high power consumption to a very low or zero power consumption. This extension of or provision of a large load range should preferably be done in a cost effective manner, with a high efficiency and low additional losses, and require few, small, readily available, and cheap components as the available space can be quite restricted. The additional regulator or regulators that are needed should preferably also be rugged, be a complement to and not negatively influence any existing regulator or regulators, and not require any special or additional supply for their function.
There exists a multitude of different voltage regulators that can be used to regulate unregulated or coarsely regulated voltages. There are conventional series regulators and switched regulators just to mention a few. The disadvantages with conventional series regulators are that they have a relatively low efficiency and normally need some kind of overload protection. Switched regulators have a higher efficiency than conventional series regulators but they usually need more components such as magnetic components that are bulky and expensive and they usually also need overload protection of some kind. On top of that, switched regulators, due to high switching frequencies, can cause interference that is difficult to moderate.
Most conventional regulators have in common that a faulty component, i.e. the regulator is malfunctioning, can lead to an output voltage that is too high, i.e. harmful, or an output voltage that is nonexistent. Regulators can be constructed so that their outputs are protected from emitting harmful voltages, but this will generally, when the protection becomes active, lead to an unusable output or a completely unusable power supply instead.